This invention relates to hydroxyl group-containing binders in the form of a non-aqueous dispersion, to their preparation and to their use for the production of multi-layered coatings. The binders are suitable both for the preparation of base lacquers and for the preparation of clear lacquers.
Non-aqueous dispersions are used in lacquers for increasing the solids content available for processing (both in base lacquers and in clear lacquers) and for improving the processing reliability (in base lacquers and clear lacquers) and the resistance of base lacquers to dissolving, without deleteriously affecting the metallic effect in the lacquer film.
Dispersions are particles of synthetic polymers in an inert liquid in which the polymer is insoluble. Known dispersions are stabilized by means of a polymeric stabilizer. The stabilizer generally contains an (anchoring) component which associates with the particles of the disperse polymer.
The dispersion contains a solvated component which forms a sterically stabilizing shell round the particles. In addition, the stabilizer is covalently bound to the particles of the polymer.
The dispersion contains further functional groups which together with other film-forming resin components (OH functional alkyd, polyester or acrylic resins) may be thermally cross-linked with conventional hardener components (melamine/formaldehyde resin) to form lacquer films.
Such dispersions are described in DE-OS 22 62 463. A stabilizer solution consisting of a precursor and prestabilizer component is used for stabilization. The stabilizer is in addition covalently bound to the disperse phase. The dispersion is prepared in aliphatic hydrocarbons. The solids content of the dispersion is about 45%. It is used together with conventional film-forming HO-functional resins and cross-linker resins in base lacquers and clear lacquers.
Dispersions on the same principle as in DE-OS 22 62 463 are described in DE-OS 28 18 095 and used there with acrylic resins as film-forming components and melamine/formaldehyde resins for base lacquers. The described dispersions are prepared in petroleum hydrocarbons (aliphatic hydrocarbons). The dispersion has a solids content of 52% and the proportion of insoluble microparticles is 31%.
Dispersions containing polyesters as film-forming components in base lacquers are described in DE-OS 28 18 093. The solvents are aliphatic hydrocarbons. The dispersions have a solids content of about 46% and the solids content of the disperse phase is about 28%. According to DE-PS 17 20 417, stabilization of the dispersion is carried out with methylol/melamine resin condensation products which undergo partial cross-linking with the acrylic copolymer. The dispersion is prepared in branched chain aliphatic hydrocarbons. The solids content of the dispersion is maximally 53%. The solids content of the disperse phase is not mentioned.
In DE-PS 22 60 610, an acrylic copolymer in aliphatic/aromatic mixtures (crystal oil) and n-propanol is described as stabilizer solution. The partial cross-linking is carried out with ethylene glycol dimethacrylate. The solids content of the dispersion is 55-60%; the solids content of the disperse phase is distinctly lower.
U.S. Pat. No. 4,525,499 describes a nonaqueous binder dispersion containing a modified vinyl polymer, or copolymer, having a hydroxyl number of between 8 and 140. The binder compositions of that U.S. patent provide acceptable stability only when used with an aliphatic hydrocarbon that is a nonpolar or only slightly polar solvent. Aromatic solvents can be used only to a slight extent lest they impair the stability of the dispersion.
Common to all the dispersions described is that the stability depends on numerous conditions which must all be observed at the same time: Use of a special stabilizer component which must in part be prepared in several stages for synthesis; use of a very limited choice of solvents or, in the case of mixtures, a narrow range of fractions, preferably aromatic hydrocarbons; copolymerisation to form the dispersion with concomitant partial cross-linking of the stabilizer/disperse phase and partial cross-linking in the core must be correctly adjusted to one another.
The aforementioned very limited choice of solvents means that only nonpolar or at most very slightly polar solvents can be employed. Even at that, the slightly polar solvents can be used only in very limited amounts lest they compromise the stability of the dispersion. This excluded from use the preferred group of highly polar solvents.
Slight errors and deviations in the parameters employed in the process of preparation may result in instability of the dispersion.
Examples of these include uncontrolled polymer formation in the process of preparation in the reactor in the region of gas/liquid interfaces, increases in viscosity leading to-loss of the high solid character or the formation of lumps leading to losses in solids content, or thickening or gelling, which may render the dispersion completely unusable.
Another indication that a dispersion is unusable is the undesirable irreversible or, in more favourable cases, reversible settling of the dispersion after prolonged storage.
It is an object of this invention to provide a nonaqueous dispersion which is suitable as hydroxyl group-containing binder, has improved stability in a wide spectrum of solvents conventionally used for lacquers, and can be produced reliably and economically.
This problem is solved by a non-aqueous, hydroxyl group-containing dispersion which is suitable for use as binder and contains, in addition to organic solvents:
A) from 15-85% by weight of film-forming hydroxyl group and carboxyl group-containing polyesters or hydroxyl group- and carboxyl group-containing (meth)acrylic copolymers or mixtures thereof dissolved in the organic solvents, in which polyesters or (meth)acrylic copolymers up to half the molecules thereof may be esterified with one molecule of maleic acid each, and PA1 B) from 15-85% by weight of a disperse phase consisting of uncross-linked (meth)acrylic copolymers having a hydroxyl number, calculated as mg of KOH per g of componet B), of at least 300, obtainable by the free radical polymerisation of PA1 in the film-forming, dissolved component provided, the sum of components A) and B) and the sum of components a) and b) amounting in each case to 100. PA1 A) from 15-85 parts by weight of film-forming hydroxyl group-containing and/or carboxyl group-containing polyesters or hydroxyl group-containing and/or carboxyl group-containing (meth)acrylic copolymers or mixtures thereof, in which polyesters or (meth)acrylic copolymers up to half the molecules thereof may be esterified with one molecule of maleic acid each, is introduced into the reaction vessel and PA1 B) from 15-85 parts by weight of monomers capable of free radical polymerisation are polymerised in this solution. PA1 a) from 80-100% by weight, preferably 90-100% by weight of hydroxyalkyl (meth)acrylates and PA1 b) from 0-20% by weight, preferably 0-10% by weight of other unsaturated monomers.
a) from 80-100% by weight of hydroxyalkyl(meth)acrylates and PA2 b) from 0-20% by weight of other .alpha.,.beta.-monounsaturated monomers which react neither with A) nor with a) by polyaddition
It was discovered that the nonaqueous, hydroxyl group containing dispersion of the present invention overcomes the drawbacks of prior art binders, in that it enables the use also of the preferred group of solvents and solvent systems. The critical feature of the present invention which enables the obtention of this benefit, is the minimum 300 hydroxyl number of the component B. It is this critical feature of the present invention which permits the use of any kinds of solvents, regardless of their polarity, and it is not recognized by the prior art.
Solvent systems having a high polarity, including esters such as a butylacetate, e.g. n-butylacetate, glycol ethers, glycol ether esters, ketones, aromatic hydrocarbons, and alcohols, and mixtures of solvents suitably of highly polar character, are the most suitable solvents that can be used with the present invention. These solvents cannot be used with the nonaqueous binders of the prior art.
For the preparation of the dispersion according to the invention, a solution of
These radically polymerisable monomers may be present as a mixture of
The monomers are introduced continuously into the solution of film forming agent present in the reaction vessel and the polymerised therein in the presence of radical initiators.